1. Field of the Invention
The present invention relates to a power supply apparatus, particularly to a power supply apparatus that implements a power saving mode.
2. Description of the Related Art
Recently, electric power saving of electronic apparatus advances. For example, there has been proposed a configuration, in which an image forming apparatus such as a printer, a copying machine, and a facsimile machine transfers a power saving mode to suppress a power consumption as much as possible in a standby state that image formation is not performed (for example, see Japanese Patent Application Laid-Open No. 2001-188442). In the power saving mode of the image forming apparatus, a power is cut off to a unit that is not necessary to be operated when the image formation is not performed. For example, the power is cut off to a motor control unit that controls a motor to convey a recording sheet. A control CPU switches an output signal to a low level to put a switch element connected through a transistor into a non-conduction state, thereby cutting off the power to the motor control unit. Therefore, the power consumed by the motor control unit is reduced. The power is cut off to units that are not necessary to be operated in addition to the motor control unit. Thus, when the image formation is not performed, the power saving mode can be implemented by cutting off the power to the unit that is not necessary to be operated.
However, in the related art, the following problems are generated in the state of the power saving mode, in the case where a cord is removed from an outlet of the apparatus or in the case where a power switch is turned off. Few charges accumulated in a smoothing capacitor provided in a power supply output of each of plural DC/DC converters in a power supply apparatus are discharged, and it takes a long time to decrease a voltage. With an increase in output capacity of the power supply because of high functionality of the apparatus, a capacitance of the smoothing capacitor tends to increase. On the other hand, because recently the power consumption is reduced to a value of 1 W or less during the power saving mode, for example, several seconds to tens seconds are required to discharge the smoothing capacitor in the image forming apparatus. In the power saving mode, when the apparatus is unplugged or when the power switch is turned off, it takes a long time to decrease the voltage of the power supply output, which results in the following problems.
A first problem in the case where an overvoltage protection circuit including an operational amplifier is used in the power supply apparatus will be described. The operational amplifier outputs a low-level signal when a voltage (+) at a non-inverting input terminal becomes smaller than an inverting input voltage V (−), and the low-level signal is input to an insulation type DC/DC converter to stop an oscillation of the insulation type DC/DC converter (see FIG. 3). In the state that the image forming apparatus transfers to the power saving mode, when the power is cut off from the power supply such that the image forming apparatus is unplugged or such that the power switch is turned off, a voltage drop at the inverting input terminal V(−) (=V3−Vf) is delayed compared with a voltage drop at the non-inverting input terminal V(+). FIG. 8 is a graph in which a horizontal axis indicates an elapsed time while a vertical axis indicates a power supply voltage and an output of the operational amplifier. At this point, V1 is a voltage in which a DC voltage obtained by rectifying a commercial power source is stepped down by the insulation type DC/DC converter, V3 is a voltage in which the voltage V1 is stepped down by a non-insulation type DC/DC converter, and Vf is a forward voltage of a diode provided in an overvoltage protection circuit. As illustrated in FIG. 8, a relationship of V(+) <V(−) holds although an overvoltage state is not generated in the power supply of the voltage V3 during the voltage drop, and the output of the operational amplifier becomes the low level to forcedly stop the oscillation of the insulation type DC/DC converter. Even if the apparatus is powered on again by plugging the apparatus by inserting a cord into the outlet or by turning on the power switch, the oscillation of the insulation type DC/DC converter cannot be started until the discharge of the voltage V3 is completed (start-up impossible time). The start-up impossible time becomes a system-down time for a user, which degrades usability.
A second problem is generated in a configuration in which the power consumption is suppressed by partly stopping a function of a control unit while the power is supplied to the control unit when the power saving mode is set. In the configuration, when the apparatus is unplugged during the power saving mode or when the power switch is turned off during the power saving mode, it takes a long time to discharge the smoothing capacitor similarly to the first problem. Therefore, an additional memory can be inserted and removed in the activation state although the power of apparatus is turned off, and possibly the additional memory and the control unit may be broken. Additionally, the insulation type DC/DC converter continues the operation until a charge voltage of the smoothing capacitor becomes a predetermined voltage or less. Therefore, even if the apparatus is unplugged while the power saving mode is set in the apparatus, or even if the power switch of the apparatus is turned off while the power saving mode is set in the apparatus, it takes a long time to stop the operation, and therefore the wasted operation is possibly generated.
In view of the foregoing, the invention enables the capacitor to be rapidly discharged even if the power is cut off from the power supply during the power saving mode.